Mast support advice

[wavelet]

Found stainless steel tubing to replace the twin steel posts beneath the mast tabernakel. Reason to change is clutter of twin posts and therefore preference for central pole in direct line with the mast as more elegant and clearing space, with polished stainless steel looking cleaner than painted steel in the cabin.
Advice on strength needed as tube is slightly thinner than anticipated. I had decided a 75mm diameter stainless steel tube of wall thickness 4 or 5 mm would be ample. The boat weighs two and a half tons, she is a Folkdancer 27 designed by Fred Parker. The tube I found has a wall thickness of 2mm (75mm) and I do not quite feel it is strong enough to take the compression of the mast. I was imagining a force of up to three tons coming through the deck from the mast, so estimated a four ton minimum compression resistance of the new tube.
Is the 2mm wall thickness a bit thin? Must I search for a heavier tube?

 

[Piddy]

The mast support on my Moody 376 has a 4"- ish stainless cover tube over a heavy duty tube hidden inside. I've no idea what the inner tube is made of but the arrangement looks OK.
That may be a way out of using more expensive stainless tube.
Cheers

 

[wavelet]

Interesting? I thought about filling with concrete, or an oak post, but then the worry of stainless steel corrosion occuring as result of being separated from air circulation, that crystaline breakdown of immersed stainless steel which I am uncertain is caused by oxidation or some other structural breakdown.
Certainly a steel pole inside the stainless would give massive strength increase.
Thanks.

 

[Stork_III]

Compression Resistance of 75 dia x 2 mm wall tube will be about 7 tonnes working load.

But would not advise 2 mm, too easily damaged. I would advise 3.5 mm minimum.

 

[William_H]

Just a slight concern that of course the thrust may not be totally in compression when well heeled.(especially if the rigging wires stretch) The original design may have been better with 2 poles if they were abeam one another and possibly spread at the bottom. A single pole would be relying on the cabin top keeping the mast well located sideways as any movement could exacerbate the side pressure. Just something to consider. For all that your 3 tons pressure may be a bit excessive. olewill

 

[wavelet]

Stork III and William H, thanks very much for ideas. I agree the resistance would rapidly decrease if the tube was dented or stressed by bending in any way. The 7 ton is much higher than I thought but also the twisting forces of heeling may alter the dynamics of compression. My present idea is go and get another tube at thicker wall and probably 100 mm diameter, then I can forget about worries completely which is more than half the battle.